1. Field of the Invention
The present invention relates to a method for producing a silicon single crystal ingot for, for example, a silicon semiconductor and the like, by Czochralski method.
2. Description of the Related Art
The apparatus as shown in FIG. 1 has been conventionally used for producing, for example, a silicon single crystal by Czochralski method (hereinafter occasionally referred to as CZ method). In FIG. 1, a quartz crucible 1 contains a silicon melt 2, and can rotate and move vertically along a crucible-holding shaft 3. A servomotor 4 drives the crucible, of which the number of rotation can be controlled. A cylindrical heater 5 made of, for example, graphite, is arranged around the crucible 1. The heater 5 has slits 20 formed vertically, and from the top and from the bottom by turns, namely zigzag, as shown in FIG. 2 and the zigzag arrangement of slits composes heat generator. The heater 5 is supplied with electricity through electrodes 12, which can be moved vertically with a driving motor 13, so that the heater 5 can be moved vertically to control the relative position of the heater to the crucible. A heat-insulating cylinder 11 is arranged around the heater 5.
Recently, there is occasionally performed MCZ method wherein a magnetic field is applied to a melt as Czochralski method. As shown in FIG. 1, a magnetic field generator 7 comprising a permanent magnet or an electromagnet is arranged outside a chamber 6 of the apparatus. Numeral 8 indicates a seed crystal 8 consisting of a single crystal silicon. The seed crystal and a single crystal ingot can be pulled with a pulling mechanism 9 with rotating around a center axis thereof. While the crystal is pulled, inert gas is introduced from a gas inlet 14 provided at upper part of the chamber 6 and exhausted from an exhaust hole 15 provided at lower part of the chamber 6 with a vacuum pump 16, and thereby amounts of inflow and outflow of the introduced gas are controlled to control an atmosphere pressure in the furnace. The silicon single crystal ingot 10 is thus produced by CZ method.
It is well known that not only components of the raw material but also the constituent ingredient of the crucible which contains the raw material for crystal growth, for example, a quartz crucible (for example, oxygen) are mixed in the crystal thus produced, while the crystal is grown by the CZ method.
The amount of the impurities which are mixed in the crystal depends on the number of rotation of the pulled crystal (rate of rotation), the number of rotation of the crucible, temperature distribution in raw material melt, gas atmosphere in the furnace, or the like. This is because the number of rotation of the crystal affects convection of the melt or an amount of impurities which are mixed in the crystal, the number of rotation of the crucible affects convection of the melt and a concentration of impurities itself in the melt as a result of change in a dissolving rate of the crucible, the temperature distribution in raw material melt, especially a relative position of the heater to the crucible affects convection in the melt. The gas atmosphere in the furnace affects an amount of evaporation of impurities from the surface of the melt. Accordingly, the concentration of the impurities in the crystal can be controlled by controlling these factors.
In recent years, requirements for a single crystal material have been getting more severe because of increase of precision and increase of integration degree of a device such as a semiconductor. It is known that impurity concentration in crystal, for example, the concentration and the distribution of oxygen in a semiconductor silicon single crystal greatly affect characteristics of a semiconductor device obtained from the silicon single crystal. Namely, too high oxygen concentration results in generation of crystal defects and oxide precipitates, which may lead to various harmful effects on characteristics of a semiconductor device. However, when such crystal defects or oxide precipitates is generated outside an active area of the semiconductor device, they act as a site of gettering heavy metal impurity, and can improve characteristics of the semiconductor device (Intrinsic gettering). Accordingly, generally too low oxygen concentration does not lead to improvement of characteristics of a device either.
Accordingly, it is necessary that intended impurities are present in crystal material in a proper amount, namely neither too much nor too little. The acceptable range of the concentration has been getting very narrow. Simple control of the above mentioned factors may result in large dispersion of the concentration, and therefore desirable concentration of impurities cannot be achieved, and thus it is not sufficient for controlling a concentration of impurities in a crystal strictly, or for satisfying the requirements. Particularly, in the case of mass production of single crystal using a lot of pulling apparatuses, a large dispersion of the impurity concentration may be generated, even though crystals are grown in completely the same condition.